Stretching From The Blue To The Green
Green lasers can serve many applications. Thanks to an emission wavelength that is
close to the peak sensitivity of the human eye, they are ideal for aiding positioning and leveling tasks in the construction industry. If they are united with red and blue sources, they can form full color projectors that can form an image on a screen. And they can also be used for medical treatments, such as addressing a common form of blindness.
The ideal laser for all these tasks is a single, small compound semiconductor chip. These are not commercially available today, but lasers based on the nitride family of materials have recently reached these wavelengths, and they promise to combine a very stable output over a wide temperature range with fast modulation speeds and high efficiency. These attractive attributes have driven the development of longer wavelength nitride lasers for many years: the 405 nm lasers that are employed in BluRay players and recorders were commercialized in 2000; and chipmakers were selling 440-460 nm lasers by the middle of that decade. However, further commercial progress has been incremental, and the longest wavelength nitride laser that can be ordered from a manufacturer emits at 488nm .
If commercial nitride lasers can be extended to the green, they could replace the far more cumbersome designs that are currently in use. These are either based on diodepumped solid-state lasers, or frequency-doubled semiconductor lasers. Regardless of what approach is used, these commercial offerings tend to be complex systems requiring additional electronics and optics for stabilization within a reasonably broad operating range.
Two common examples of the diode-pumped solid-state laser are neodymium-doped yttrium aluminum garnet (Nd:YAG) and neodymium-doped yttrium orthovanadate (Nd:YVO4) lasers. Both operate by employing a 808 nm GaAs-based laser diode to optically pump a 1060 nm solid-state laser, and then frequency doubling the emission produced by this laser to 530 nm with a nonlinear optical up-conversion process, using lithium niobate (LiNbO3), lithium triborate (LiB3O5), or similar crystals.
Research engineers at Osram Opto Semiconductors have recently developed a series of green InGaN laser diodes that emit at wavelengths up to 526nm
If the temperature of the pump diode shifts, its emission wavelength changes considerably. This is highly undesirable because the solid-state laser has a narrow absorption range, and this in turn restricts its operating temperature, which can be increased through the costly addition of external stabilization. To make matters worse, there is a strong intensity noise issue that can require additional elements within or outside the laser cavity for stabilization. Another downside of this type of laser is that its modulation speed is limited to the kHz range, due to the long lifetime of the charge carriers, and this means that the diode-pumped solid-state laser cannot be deployed for scanning beam projectors without an additional external modulator. However, in its very simple configuration this type of lasers can be used as a laser pointer, and more complex versions delivering a stable output power can serve medical and bio-technical applications. And by increasing the output power, these emitters can also be used in laser shows.
The other form of commercial green laser features either intra-cavity or extra-cavity frequency doubling. Both variants deliver fast switching times thanks to a 1060 nm, GaAs-based semiconductor laser source that is either electrically or optically pumped. Stable laser output over the operating temperature range results from maintaining the semiconductor emission at the frequency conversion wavelength.
This can be realized by adding a DBR structure to the electrically pumped laser, or using filter elements for the optical pumped laser. For intra cavity frequency doubling a periodically poled LiNbO3 bulk crystal can be used. For the conversion outside of the cavity a several mm-long periodically poled SHG-crystal with additional narrow channel waveguide structure is used in order to reach the necessary power density for efficient laser light conversion.
Intra-cavity and extra-cavity approaches can produce MHz modulations speeds and operate over a wide temperature range. However, complex driving electronics are needed to drive these lasers efficiently.
Fig 1. Osram s 520nm ridge waveguide laser, which has 2um broad stripe with 600um resonator length, can deliver an optical output power of 50mW in pulse operation. To limit thermal effects the laser was measured in pulse mode with a duty cycle of 1 percent and a pulse length of 1us
Fig 2. Ridge laser emission can support several lasing modes at 520nm in pulsed operation at 50mW output power at room temperature
The problems with nitrides
If nitride lasers are to replace the more complex diodepumped, solid-state lasers and frequency doubled semiconductor lasers, then researchers must overcome the challenge of routinely producing high-quality InGaNquantum wells with sufficiently high indium content. Extending emission from the blue to the green demands an increase in indium content in the InGaN wells, but this is hampered by a deterioration of the thermal stability of this layer, alongside higher compressive strain due to larger lattice mismatch [2, 3].
Our team of researchers at Osram has made an important breakthrough in this area - we were the first to break the 500 nm barrier . Since then we have progressed to even longer wavelengths, including the realization of 515nm and 520 nm pulsed laser operation from broad, gain-guided test laser structures [5, 10]. Other researchers have also enjoyed success, such as Nichia, which has produced 8 mW continuous-wave operation at 515 nm from small ridge waveguide lasers.
Our lasers, and those developed by Nichia, are produced by growth on the c-plane of gallium nitride. One weakness of this approach is that high internal piezoelectric fields in the polar growth direction hamper the device performance. These fields can be either minimized or eliminated by turning to semi-polar and non-polar planes, respectively. Rohm has adopted this approach, and reported the longest lasing wavelengths for a laser on a non-polar m-plane GaN-substrate. It has developed a 499.8 nm laser with a very high junction temperature and 97% out-coupling mirror reflectivity . Progress has also been realized by Sumitomo, which announced a 531nm broad gain guided test laser in summer 2009 that was driven in pulsed operation and grown on the semi-polar plane . Later that year this company reported continuous-wave operation up to 2mW from small ridge waveguide laser at 520nm on -plane . Regardless of the growth plane, producing high material quality with a low defect density is a big challenge.
More recently we have managed to push our lasers to even longer wavelengths, and realized 526 nm emission from broad-area, test laser structures on c-plane GaN. Additional results include a 520 nm ridge waveguide laser with a record optical output power of 50 mW in pulse operation (Figs. 1 and 2). This optical power level is suitable for second-generation, red-green-blue scanning projection technology that can deliver an illumination level of about 10 lumen on the screen.
Our direct green 520 nm ridge laser was processed as a 2 μm broad stripe with a 600 μm resonator length, and it includes natural, cleaved laser facets with dielectric mirror coatings. The epitaxial structure consists of AlGaN cladding layers, GaN waveguide layers and an active region containing InGaN layers with different indium content on a c-plane GaN-substrate (Fig. 3). The laser characteristics include a threshold current of about 125mA, which is only four times the threshold level of currently available blue InGaN laser diodes.
Several hurdles had to be overcome to extend our lasers to 500 nm and beyond. Probably the biggest of these was improving the crystal quality of the high-indiumcontent quantum wells needed for green emission. The quality of this layer can be assessed with microphotoluminescence mappings (fig. 4). The black spots in the left image of fig. 4 are areas of weaker green spontaneous photoluminescence emission, due to segregation of indium atoms.
Fig 3.Osram s green-emitting laser is a conventional ridge laser design that includes GaN waveguides, an AIGaN electron-blocking layer and AIGaN cladding layers
A strong correlation exists between the formation of low spontaneous emission areas and high densities of nonradiative defects. The right image in fig. 4 shows an incredibly uniform green photoluminescence from a laser structure with higher crystal quality. Employing improved growth parameters and designs on c-plane GaN substrates formed this structure. No black spots can be seen, indicating improved crystal quality. This material produces devices with a higher peak gain for lasing.
The behaviour of our lasers is influenced by the strong piezoelectric fields within the quantum wells. The straininduced piezoelectric fields and tilted energy potentials reduce the band gap. This is the so-called Stark-Effect. The lower band gap helps to reach the long emission wavelength without changing the material composition. However, lasers operate at current densities that are typically orders of magnitude higher than LEDs, which partially screen the internal fields, leading to a blue shift of the laser emission wavelength (Fig. 5 right). However, if the laser threshold current can be reduced, the Stark- Effect in the polar growth direction can be used to shift the laser towards longer wavelengths without increasing the indium-content in the InXGa1-XN quantum wells.
Fig.4. Micro-photoluminescence mappings reveal the quality of the quantum well layers. Left: Lower material quality, including dark spots due to indium-segregation. Right: Uniform green photoluminescence emission of InGaN quantum wells with higher material quality
Alternative approaches to reducing the internal built in piezoelectric fields are based on semi-polar or non-polar GaN substrates. However, lasers grown on non-polar mplane GaN substrates currently suffer from poor material quality of the high indium-content layers needed to produce green emission.
Fig. 5 Left: The piezoelectric fields in polar material alter the conduction and valence band profiles of the within InGaN-quantum wells (left). Carrier separation at the quantized band states is indicated by the calculated carrier envelopes, which show a reduced electron-hole overlap compared to a rectangular quantum well profile (not shown here). Right: Increasing the current density of a polar green InGaN-based LED to values needed to drive a laser produces a large blue-shift in the spontaneous emission wavelength. Nitride lasers with similar indiumcontent in the quantum wells that are grown on non-polar substrates produce a far smaller variation in emission at shorter wavelengths (dashed line)
Although there are issues affecting all forms of nitride laser, this should not obscure the tremendous progress made with green lasers over the last year or so - it is now possible to produce directly emitting green lasers at wavelengths of 515nm and beyond on polar and semipolar planes. Future goals include improvements in the material quality of the quantum wells. If success follows, small laser chips could replace the bulky, complex devices currently used to provide a green laser source.
The German Federal Ministry for Education and Research (BMBF) supports the research activities on blue and green InGaN lasers within the project MOLAS (FKZ 13N9373)